Insights on the evolution of the Himalayas and Tibet from thermomechanical modelling: the role of long-term convergence

The collision of India and Eurasia has resulted in a broad range of structures, from the Himalayan chain to the Tibetan Plateau. The convergence history is characterised by velocities of > 10 cm/yr at collision to current velocities of ~5 cm/yr, of which ~2cm/yr are accommodated at the orogens' front. Our thermomechanical model simulates the collision of India and Eurasia to assess the role of the decrease in velocity, highlighting 4 key phases in the evolution. Phase 1 (50 - 44 Ma) is characterised by a proto-wedge formation, whilst in phase 2 (44 - 32 Ma) the large convergence velocity drives exhumation of crust along a localised channel flow at the front of the wedge, and underthrusting of Indian crust to form a plateau. As convergence slows in phase 3 (32 - 25 Ma), temperature in the orogenic root increases, while compression decreases, allowing the buoyant resurgence of deep crust to form a shallow crustal dome. Further slowing in phase 4 (25 - 0 Ma) allows some hot crust emplacement beneath the plateau. The shallowing of the thermally-activated brittle-ductile transition causes fold and thrust belt (FAT) development at the front of the wedge, while deep burial and exhumation cease. When compared to nature, the deformational phases and their duration are remarkably matched. A switch from large convergence phases, dominated by deep burial, localised exhumation and crustal under thrusting, to slow convergence phases, characterised by large-scale doming of buoyant crust and the formation of a FAT belt, is observed.